An important challenge in nanoscience is the characterization and analysis of structures that are assembled on technologically relevant substrates. A number of scanning probe and electron beam-based microscopies have been employed, each possessing unique advantages, complexities and substrate requirements. Imaging performance is typically enhanced through the use of specialized substrates. Unfortunately, such substrates are often chemically dissimilar to the substrates used in devices or during assembly reactions. For example, a carbon-coated TEM grid has different surface chemistry than a semiconductor wafer.
The most commonly used methods for analyzing nanostructures on chemically functionalized surfaces are atomic force microscopy (AFM) and scanning electron microscopy (SEM) because these techniques are compatible with a wide range of substrates including SiO2. In a direct comparison of AFM, SEM, scanning near-field optical microscopy (SNOM) and transmission electron microscopy (TEM), Grabar et al. demonstrated that TEM is a preferred method for quantifying the size, shape, and spacing of nanoparticles in nanoparticle arrays due to high lateral resolution and straightforward data analysis. Grabar et al., Anal. Chem. 1997, 69, 471-477. The primary limitation of using TEM to analyze nanostructures is that the relevant substrate material may not be available as a support film on commercially available grids. In order to obtain images of samples on relevant substrates, time-intensive, destructive sample preparation techniques such as mechanical polishing or ion milling must be employed in order to obtain electron transparency.
Commercially available silicon monoxide (SiOx) TEM grids are often used as approximants for SiO2 surfaces. These substrates generally consist of a metal grid coated with a polymer support that is coated with a substrate material such as SiO or carbon. Unfortunately, these substrates are rough, lack rigidity, and the SAX surfaces have an ambiguous chemical structure that is a mixture of SiO and SiO2. Therefore, such surfaces do not have the same chemical reactivity as native or thermally grown SiO2 on silicon. Due to the reactivity of the polymer coated metal grid that supports such SiO films, these grids cannot withstand even the mildest environments that are used for cleaning and processing SiO2/Si. UV/ozone cleaning destroys the polymer support, as do RCA SC-1, piranha solution, and oxygen plasma, while RCA SC-2 or other acidic environments will dissolve most metal substrates. In addition, the chemical environments used to functionalize SiO2, such as self-assembled monolayer chemistry, often involve acidic environments and organic solvents. The ideal TEM grid for imaging SiO2 surfaces must be electron transparent, smooth, rigid, and robust to chemical processing.
Recently, it has been shown that the surfaces of silicon nitride TEM grids can be oxidized to SiO2 by O2 plasma treatment, but the chemical nature and reactivity of such surfaces has not been determined. See Grant et al., Nanotechnology 2004, 15, 1175. Because thermal oxides react differently to surface treatments than native oxides due to the nature of the surface hydroxyl groups, surfaces with a stoichiometry of SiO2 are not necessarily equivalent. Thus it cannot be assumed that the images obtained from an oxidized silicon nitride TEM grid represent the surface of a similarly treated glass slide or silicon chip.
TEM grids with electron-transparent Si3N4 windows are commercially available. While Grant et al. report that grids of this type can be oxidized in an oxygen plasma to produce an SiO2 surface, analytical data and the chemical reactivity of these surfaces have not been reported. Grant et al., “Transmission electron microscopy ‘windows’ for nanofabricated structures,” Nanotechnology, 2004, 15, 1175-1181. Kennedy et al. report that the chemical composition of oxidized silicon nitride surfaces depends noticeably on the method of oxidation, ranging from an oxynitride composition at lower levels of oxidation toward a “silicon oxide rich” layer after more extensive oxidation. Kennedy et al, “Oxidation of silicon nitride films in an oxygen plasma,” J. Appl. Phys., 1999, 85, 3319-3326. Ito et al. have reported that the reactivity of the “native oxide” on silicon nitride depends on the method of sample preparation. Ito et al., “Modification of Silicon Nitride Tips with Trichlorosilane Self-Assembled Monolayers (SAMs) for Chemical Force Microscopy,” Langmuir, 1997, 13, 4323-4332. Given the marked dependence of the surface reactivity of silicon dioxide and oxidized silicon nitride on the method of preparation (e.g. native oxide and thermal oxide exhibit different reactivity due, in part, to the differences in surface hydroxyl concentration), it remains to be seen whether oxidized silicon nitride surfaces will serve as suitable approximants for a thermal silicon dioxide surface. From the data published for the oxidized silicon nitride grids, it appears unlikely that these grids will exhibit the surface reactivity found for thermal silicon dioxide.
To scale electronic devices down to nanometer dimensions, fundamentally distinct new technologies are needed to provide smaller features that can confer heretofore unattainable electron flow control. The ultimate limit is a system in which the transfer of a single charge quantum corresponds to information transfer or some type of logic operation. Such single-electron systems are presently the focus of intense research activity. See, for example, Single Charge Tunneling, Coulomb Blockade Phenomena in Nanostructure, edited by H. Grabert and M. H. Devoret, NATO ASI Series B: Physics Vol. 294 (1992). These systems have potential application to nanoelectronic circuits that have integration densities far exceeding those of present day semiconductor technology. See, Quantum Transport in Ultrasmall Devices, edited by D. K. Ferry, H. L. Grubin, C. Jacoboni, and A. Jauho, NATO ASI Series B: Physics Vol. 342 (1995).
Single-electron transistors based on the concept of Coulomb blockade are one proposed technology for realizing ultra-dense circuits. Coulomb blockade is the suppression of single-electron tunneling into metallic or semiconductor islands. In order to achieve Coulomb blockade, the charging energy of an island must greatly exceed the thermal energy. To reduce quantum fluctuations the tunneling resistance to the island should be greater than the resistance quantum h/e2. Coulomb blockade itself may be the basis of conventional logic elements, such as inverters. Equally promising is the use of the Coulomb blockade effect to pump charges one-by-one through a chain of dots to realize a frequency-controlled current source in which the current is exactly equal to I=ef, where f is the clocking frequency.
While the operation of Coulomb blockade devices has been demonstrated, most operate only at greatly reduced temperatures and require sophisticated nanofabrication procedures. The size scales necessary for Coulomb blockade effects at such relatively elevated temperatures of about room temperature impose limits on the number, uniformity and connectivity of quantum dots. As a result, alternative methodologies of nanofabrication need to be investigated and developed.
The electronic properties of small metallic nanoparticles have been examined for application in nanoelectronics, catalysis, sensors and optics. However, few devices that incorporate such nanoparticles have been developed to date, in large part due to the inability to precisely control the anchoring and positioning of nanoparticles on a substrate. Prior approaches to nanoparticle deposition on surfaces typically have failed to provide the necessary control over nanoparticle size distribution, interparticle spacing, and/or are incompatible with semiconductor processing methods. For at least these reasons, improved electronic transmission grids that can be precisely functionalized are needed.